1. Field of the Invention
This invention relates to semiconductor processing and, in particular, to processes involving vacuum deposition of semiconductor materials.
2. Art Background
Often the particular sample holder employed in vacuum deposition processes, i.e., process performed at pressures less than 1 Torr is of little consequence. Generally minimal, if any, effort is devoted to the design of innovative sample holders for procedures involving the vacuum deposition of semiconductor materials from the vapor phase. However, in techniques such as molecular beam epitaxy (MBE) where extreme purity and crystal quality is generally desired, the specific substrate holder employed often makes a significant difference in the quality of the deposited material. Generally, for such applications it is desired that the substrate holder introduces only negligible strain into the substrate upon which deposition is to be performed. Additionally, the sample holder is also designed so that it does not substantially introduce contamination, e.g., impurities, into the deposited material.
A sample holder prevalently employed in MBE involves the use of a heater block formed from a refractory metal such as molybdemum and having a thickness which is large compared with that of the substrate. An intermediary material with a low melting point, e.g., indium, gallium or an alloy of indium and gallium is applied onto the block, and the entire structure is heated until the intermediary material melts. The substrate is then pressed against the molten intermediary material which wets and conforms to the substrate surface. During deposition the intermediary material remains molten, and the substrate is held by surface tension so that little strain is introduced. The molten intermediary material allows efficient heat conduction and the thick heater block assures uniform temperature distribution. The intermediary material, e.g., indium or gallium, generally does not contaminate the deposited material.
Despite these advantageous properties, the use of a refractory block in conjunction with a material such as indium or gallium does present certain difficulties. The mounting and dismounting of the substrate is time consuming and requires skillful execution to ensure uniform spreading of the intermediary material and to minimize trapping of gas between the substrate and the intermediary material. Additionally during deposition, gallium or indium forms an alloy with the III-V materials of the substrate. After deposition, the substrate is separated from the heater block while the intermediary material is in the molten state. However, after this separation, some of the intermediary material remains and regions are present which are formed by the interaction of the substrate and the intermediate material, e.g., in the case of a GaAs substrate, alloy regions of Ga/In and hillock shaped regions of InAs. These residues are subjected to a chemical etch which removes the remaining intermediary material, which also removes the alloy thus forming etch pits, and which leaves hillock regions such as InAs regions unaffected. The hillocks and pits cause significant problems in subsequent processing. For example, the hillocks interfere with the handling of the substrate by conventional equipment such as vacuum chucks. The parallelism of the top and bottom surfaces of the substrate is degraded producing lower yield during subsequent lithographic processing. Additionally, the uniform thinning of the substrate is often difficult since areas that originally contained pits are removed substantially before other regions.
The use of a relatively thick heater block also involves certain consequences that are not totally desirable. For example, the large thermal mass of the block prevents rapidly changing the temperature of the substrate during deposition. A rapid temperature change, however, is often desirable in situations where heterostructure growth is involved.
The alternatives to the use of an intermediary material, such as indium or gallium, are even more undesirable. The use of spring clips or other contact devices which apply pressure to provide thermal contact between the substrate and the heater block generally introduces an unacceptable level of strain which produces undesirable effects such as lower charge carrier mobility in the deposited semiconductor material. Further, the use of spring clips does not produce a uniform contact between the substrate and the block. Thus heat reaching the substrate must, in this situation, predominantly through radiative, not conductive, transfer. (Convection is precluded since the deposition is performed at low pressures.) However, typical III-V semiconductor substrates such as GaAs and InP of nominal thicknesses, 250 to 750 .mu.m, are substantially transparent to infrared radiation. Thus, in the absence of an intermediary material such as indium for heat conduction, heating is quite inefficient since little radiative energy is absorbed. The additional heat required by this inefficient heat transfer produced additional outgassing of impurities which, in turn, adversely affects the quality of the deposited layers. Thus, although present sample holder configurations are often acceptable, improvement is certainly desirable.